Stanford

Around the Bay

Matter-Antimatter Oscillations

Matter changing spontaneously into antimatter and vice versa may sound like science fiction, but we use it all the time at the B Factories. Such
"oscillations" are allowed in the Standard Model and have been observed for neutral mesons containing strange (the
K0 meson containing a strange antiquark and a
d quark) or beauty quarks (both the B0 meson containing a beauty antiquark and a
d quark). In the summer of 2006, the process was also seen by CDF and D0 for the neutral
B0s meson, containing a beauty antiquark
and a strange quark (read the press release
here). These spontaneous transitions occur in the weak interaction through virtual quantum loops involving two virtual
W bosons and two virtual quark lines, dominated by virtual top exchange. With four legs, containing virtual
W or quarks, the diagram governing matter-antimatter oscillations can be drawn to look like a box, which is the name it goes by. The oscillation
B0àB0 and
B0àB0 actually competes quite favorably with direct decays, so that matter-to-antimatter transitions happen with about 20% probability before decay. Indeed, it is this oscillation phenomenon that allows the whole program of CP violation studies in the B meson system. The original observation of
B0àB0 oscillations, a virtual loop process sensitive to physics at much higher masses, was an early indicator, long before its direct discovery, that the top quark was indeed very heavy. In the case of neutral mesons containing a charm quark (the
D0 meson containing a charm quark and an anti-u quark), the Standard Model process is very suppressed. This makes searches for the matter-antimatter oscillation process in the neutral charm meson system a sensitive probe for new physics contributions to the box diagram and a hot area of study.